The present invention relates to methods and devices for applying coatings by cylindrical magnetron sputtering, in particular devices and methods where a magnetic field is created by an assembly of electromagnets.
Magnetron sputtering processes are classified as planar or cylindrical. The planar (circular, rectangular and triangular shaped) magnetron sputtering devices generally suffer from non-uniform erosion, with the area of maximum erosion in the shape of a racetrack centered around the magnet position, rendering the target unusable after use, even while relatively large amounts of useful target material still remain. Also, planar magnetrons often employ large magnet assemblies, which are not useful for creating films inside structures with hollow workpieces having annular cavities, such as narrow diameter pipes.
Several different types of cylindrical magnetron sputtering devices have been developed, as disclosed and summarized by Thornton et al., “Cylindrical Magnetron Sputtering”, 1978 Academic Press, Inc., pp. 75-113. Cylindrical magnetron sputtering devices are used to coat cylindrical workpieces, such as the inside surfaces of pipes. Basically, the target material in a cylindrical magnetron sputtering device is in the form of an elongated tube.
U.S. Pat. No. 4,031,424, issued to Penfold, describes solenoid coil configurations used to provide a confining magnetic field around a hollow cathode having a cylindrical barrel and end flanges. In one configuration, the workpiece is placed inside the cathode and the solenoid coil(s) are placed outside the cathode. In another configuration, the workpiece is placed outside the cathode in a vacuum chamber. One solenoid coil winding is disposed about the outer wall of the vacuum chamber and another solenoid coil winding is disposed within the cathode. Both arrangements are stated to employ a magnetic field whose lines are generally parallel to the axis of the cathode barrel. Externally positioned solenoid coils are not useful for coating the surfaces of ferrous pipes since externally positioned coils magnetize the workpieces creating magnetic shields, and complicate the deposition process, especially when coating pipes with non-constant wall thickness or tapered inside diameter.
U.S. Pat. No. 4,376,025 issued to Zega orients the magnetic flux lines circularly around the axis of the elongated rod-like target material, as opposed to the axial orientation used by Penfold. Zega describes a cylindrical magnetron device utilizing a tubular current-carrying anode disposed within a tubular target cathode. The disadvantage of this approach is that, while very efficient with small diameter targets, it becomes less efficient as the target diameter increases. Further disadvantages of this high current approach are the considerable additional power input needed for the large diameter/high electrical resistance target and practical limitations of the system to uniformly coat non-cylindrical, or tapered inside surfaces of elongated components.
U.S. Pat. No. 4,407,713 issued to Zega describes a magnetron sputtering device which creates a magnetic field with an assembly of permanent magnets. The magnet assembly consists of a plurality of equiangularly spaced axially extending radially magnetized magnets arranged in such a manner that their flux lines form over the sputtering face a plurality of equiangularly spaced axially extending straight arch portions connected to each other by arcuate arch end-portions, whereby defining at least one closed-loop arch over the sputtering face. The magnetic assembly may be axially rotated relative to the target. The device is cumbersome for use in small diameter pipes and has little application for hollow workpieces having complex shapes.
Alternative cylindrical sputtering devices are disclosed in U.S. Pat. No. 4,221,652 to Kuriyama, and in U.S. Pat. No. 4,179,351 to Hawton et al. In both devices, a cylindrical cathode comprising the material to be deposited is placed within a substantially cylindrical workpiece. Within the cylindrical cathode are one or more cylindrical magnets oriented symmetrically about the axis of the cylinder for generating a toroidal magnetic field. The cathode surface is located in close proximity of the magnet poles such that magnetic field lines penetrate the cathode and form a closed ring gap. Within the cylindrical cathode also exists a pipeline for introducing a coolant. The multiple permanent magnets produce multiple toroidal volumes or particle racetracks, in which the charged particles are concentrated. This results in multiple erosion zones, each zone being centered upon the center of a magnet, rather than in a single erosion zone, as would be obtained from a single magnet. U.S. Pat. No. 4,221,652 describes movement of a single magnet along the axis of the target to obtain uniformity in thickness of a metallic film deposited on a workpiece.
The figures of JP Patent Publication No. 55-27627, corresponding to JP Patent 52095581 A, listing inventors Misumi, Takashi and Hosokawa, Naokichi also illustrate one or more magnets within a sputtering target. Movement of the magnet(s) within the target is indicated.
U.S. Pat. Nos. 4,356,073 and 4,422,916, issued to McKelvey, describe rotatable magnetron sputtering apparatus where magnetic means are mounted within a tubular cathode. The magnets described are U-shaped permanent magnets.
U.S. Pat. No. 4,904,362 issued to Gaertner et al. describes a cathode arrangement having an internal, cooled permanent magnet system. The permanent magnets inside the target are cut so that their end faces are at an angle of 45 degrees to 75 degrees to the longitudinal axis of the cathode arrangement and are magnetized so that their poles lie in the end faces. The magnets are disposed so like poles are adjacent. Rotation of the magnets causes rotation of the plasma zones. The surfaces to be sputtered are subjected to a mutual relative motion in the longitudinal direction of the cathode arrangement or carrier tube. The disadvantage of the apparatus described by Gaertner et al is use of magnets with constant, un-adjustable magnetic field and relatively complicated design of the permanent magnet assembly.
U.S. Pat. No. 6,436,252 issued to Tzatzov et al. describes a cathode assembly for magnetron sputtering of a cylindrical workpiece which assembly includes a magnet package consisting of a plurality of spaced permanent magnets of alternating polarity. The magnets may be joined with ferromagnetic spacers. The magnet package is positioned on the inside of the cathode such that a driving force applied to the magnet package or to the cathode, or to both independently, imparts relative longitudinal movement between the magnet package and the cathode.
In summary, there is a still a need for an device that can be used to provide uniform coatings on the inside surfaces of tubular components such as tapered pipes, and do so with flexibility to apply the desired levels of magnetic field to specific areas on the cathode that correspond to the specific irregular internal surface of the elongated workpiece.